Revisit <merge> nodes implementation in "uberwar" merge descriptor

Currently <merge> node accepts four sub-elements: <document>, <classname>, <type> and <parameters>.
Only specific conjunctions of those sub-elements (sometimes with specific values) actually work, the rest just fail silently.

Moreover, the implementation of <type>web.xml</type> does not create MergeProcessor at all. During processor creation phase only the configuration is read, and the work is actually done by other dedicated classes. As a result, the caller class if forced to accept nulls, ignoring potential failures in creation of other processors.